One of the blessings of computerized dentistry (CAD/CAM technologies) is that it enabled automated production of dental restoration structures of zirconia or metal alloy without intervention of a dental technician. The introduction of zirconia (zirconium-oxide) in restorative and prosthetic dentistry is most likely the decisive step towards the use of full ceramics without limitation. With the exception of zirconia, existing ceramics systems lack reliable potential for various indications for bridges without size limitations. Zirconia with its high strength and comparatively higher fracture toughness seems to buck this trend. With a three-point bending strength exceeding nine hundred mega Pascal (900 MPa), zirconia can be used in virtually every full ceramic prosthetic solution, including crowns, bridges, abutments and implant supra structures.
However, so far computerized dentistry could not provide the automatic production of structures of zirconia or metal alloy with one or more veneer layers without the intervention of a dental technician. A massive material without a veneer layer can never fulfill the aesthetic requirements of natural layered tooth. Surprisingly a method was found whereby both an automated manufacturing and an aesthetically pleasing result could be obtained.
Dental restorative systems that comprise a structural support part (a dental restoration core) and a visible part (e.g., a veneer layer) which is supported by the structural support part, seek to provide cosmetic and functional replacements for missing teeth.
A customized dental restoration should match the size, shape and contour of the original teeth in order to provide the best possible appearance. Standard methods for preparing dental restorative systems require considerable time, labor, and expense. Methods typically require that a patient makes between six and ten visits to the dentist's office to complete installation of the restorative system.
In the conventional method for the construction of a dental superstructure such as the dental restoration system, a physical model of the patient's gums and dental implant heads is prepared on which the dental superstructure is built manually using molding and other techniques known in the art. The craftsman or technician skilled at manufacturing such dental superstructures takes into consideration the size and shape of the desired dentures to be placed over the superstructure when crafting the same.
Conventionally a cast, milled or 3D free form printed core structure is veneered by the dental technician by hand using a time consuming layering technique.
Another possible production method is using CAD/CAM methods to produce a core structure by milling or 3D printing and the veneer in the form of a incinerable material such as methacrylate or polyurethane. The acrylic veneer is placed over the core, sprue channels attached and the assembly invested in a refractory investment material. By burnout of the incinerable part a veneer void is created. Next, the veneer void is replaced by hot pressing a glass ceramic above its melting temperature in the veneer void. The restoration is divested and further finished by cutting back and adding new porcelain and staining and glazing by the dental technician. Recently, another method to veneer a zirconia substructure was proposed for the production of a individual veneer cap of a glass ceramic (e.g., lithium silicate strengthened ceramic) which can be placed over the zirconia structure and whereby the veneer cap can be connected to the zirconia core structure by a low-fusing glass ceramic layer. (Beuer F, Schweiger J, Eichberger M, Kappert H F, Gernet W, Edelhoff D., “High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings—a new fabrication mode for all-ceramic restorations”, Dent Mater. 2009 January; 25(1):121-8).
Both methods still require manual steps in the production of the veneered restoration, while the manual steps in the present invention are limited to final glazing and staining of the restoration by the dental technician. Also they make use of massive materials that can never provide the same aesthetics as a multi-layered build-up.
The dental profession utilizes standardized shade guides. A well-known shade guide is the Vita™ Shade Guide, which includes sixteen different shades. However, these shade guides are utilized in a rudimentary fashion. The guide itself is a plastic plate with a plurality of removable color tabs that are shaped like a tooth e.g., a front tooth. Typically, to assess a patient's tooth shade, a dentist removes one of the colored tabs from the plate and holds it up to the patient's tooth so that he/she can visually determine the closest match possible. There is a necessity to improve color determination and simplify the handling of components for dental restorations. All major shade guides are derived from prosthesis teeth and are never meant for individual color determination of natural dentition. Color groups have been created but basically in human dentition only two color groups exist, reddish and yellowish. All deviated colors are caused by a colored transparency. Looking at the known Vita Shade Guide, only the reddish A and yellowish B group are in general, useful in determining human teeth. Shades in the C and D groups can be useful, but are only transparency variables of the A and B group. The downside of the A and B group is its inharmonious and irregular color gradient. Each color in the group has its own pigmentation and therefore a real fluent gradient is not present.
Understandably, there are many variables to this method, some of which stem from the subjectivity of the dentist making the eyeball assessment. Such shade guides have been utilized for decades and the color determination is made subjectively by the dentist.
In order to lower the subjective uncertainty a shade analyzer can be used that provides a methodology for assessing and communicating a patient's tooth color in an objective way.
The task of replacing a tooth is conventionally made of two separate steps. The first step is to measure the shape and color shade of a tooth to be replaced and the second step is to make a duplicate of that tooth according to the measurements taken in the first step.
In the first step, while the shape information can be acquired with molding technique, the measurement of the color shade and translucency of the tooth proves to be more challenging.
The quality of the dental prosthesis cannot be better than the data that serves to model the tooth. The precision of that model depends on several factors, like the quality of the illumination, the data acquisition by measuring and the processing of those data. The oldest and simplest way of determining the color shade of an object like a tooth is to compare visually the object with a chart of color shades. The results obtained with that method are however not very good because of the subjectivity of the human eye. Furthermore, the illumination of the tooth and of the chart may cause inappropriate color shade choices.
A quantitative method can be used to obtain a minimum of precision and of reproducibility in the measurement of the color shade of an object. Such quantitative methods can be classified by the type of illumination used, the measurement technique, the data processing and the comparison between the finished product and the original object.
The illumination is usually done by using fiber optics or a fiber optic bundle to illuminate the surface of the object to be measured. It is advantageous to control the illumination of the object since the characteristics of the illumination method may be taken into account during the data processing. Diffuse light provides a simple means to control illumination.
Several methods are known and used to convert the spectral decomposition or the data collected from a selected area into a single measurement that corresponds to the color perception of the human eye. The objective is to quantize the data and also to correct them as to be able to recreate the proper colors of the original model as the human eye perceives them. It is also important to be able to quantize the translucency of the materials.
In WO 97/01308 an oral camera connected to a shade analyzer subsystem, e.g. a digital video processor, and a color display monitor. The camera captures a digital color image of the patient's tooth and the subsystem compares that image to a stored plurality of tooth shades. Each tooth shade is represented in a block of data, including color image data, a tooth shade digital word, and a manufacturer type. The patient's tooth image includes an RBG chromatically representation that is scanned and compared with the several tooth shades stored in memory, and a match is determined and communicated to a user of the system. The methodology includes the specification of fractional tooth shades, if needed, corresponding to a plurality of glass ceramic forms for manufacturing a reconstructed tooth.
The information is then used by a technician to layer the crown following the identified color shade as measured and presented by the digital device. This process of fabricating a crown by a way of layering the material by hand is fairly tedious and costly as it takes much hands-on time. The result of this hand-work is unpredictable and the result depends to a large extent on the skills of the dental technician.
In EP 0796596 a system for recording the form and shade structure of teeth is described. The system is applied in the preparation and the production of ceramic or acrylic veneered restorations. The system consists of different assortments containing the models and images as well as layering schemes of different tooth form and shade structures. At the patient a comparison is made between the form and shade of the patients' teeth with the models, whereby the best fitting assortment is selected and in the dental laboratory according to the accompanying layering schemes nature-like restorations can be reproduced.
Because results of the digital color analyzer only result in prosthesis through the manual labor of the dental technician, still pseudo trial and error methods are used in the manufacturing of the prosthesis remain, with the result that prosthesis may need to be remade, leading to increased costs and inconvenience to the patient, dental professional and dental laboratory.
The main difficulty of measuring translucency and color simultaneously arises from the fact that the information of these two appearance factors is usually interlinked. Different approaches can be used to disambiguate these two appearance factors:
1. The auto-correlation functions for the three color channels provide information on the blur which can be caused by the translucency. Structured lighting can be used to increase and further disambiguate the signal.
2. Translucency can be evidenced by comparing successive images taken with alternately a white and a black background. A structured background can also be used to evidence transparency.
3. The knowledge of the color space covered by the material can also be used to parse color and translucency variations.
In most of the present existing CAD/CAM-systems dental restorations are produced from massive material blocks in the dental laboratory or in the dental practice. Machining a restoration from a uniformly colored monolithic block of material can never fulfill both the aesthetic demands and the requirements for strength.
It is an object of the present invention to provide a method for manufacturing an dental restoration. This object is achieved by a method according to claim 1.
Advantageously, the method provides the manufacture of a dental restoration in which the veneer layer is shaped on the dental restoration core without the need to define the veneer layer as a solid element before positioning on the dental restoration core. Application of a slurry or paste on the dental restoration core structure by using a mould block that has an open space that has the shape of the contour of the veneer layer surrounding the dental restoration, allows the direct formation of the veneer layer on the dental restoration core. The invention reduces the labor for layering the veneer on the dental restoration core.
It is also an object of the present invention to provide a dental restoration in which a colour of the dental restoration is controlled. This object is achieved by a method according to claim 19. Advantageously, the arrangement of a dental restoration core with a high intensity colour covered by a translucent veneer layer with a low intensity colour allows to express a resulting colour for the dental restoration composed in a first part by the colour of the core and in a second part by the colour of the veneer layer.
The colour expression is controllable by the thickness of the translucent veneer layer which determines the ratio between the first and the second part.
Advantageously, the colour expression according to the method can provide in a substantially continuous gradient, any colour between the high intensity colour and the low intensity colour since the thickness of the veneer layer can be varied continuously. This allows to have a close match between a dental restoration and the neighboring dental elements of a patient.